1. Field of the Invention
The present invention relates to a method of manufacturing a lead frame used for the manufacture of a semiconductor package, and to the lead frame.
2. Description of the Related Art
In recent years, as the integration degree and packaging density of semiconductor integrated circuits have increased, an increase in the size of the semiconductor package and an increase in the number of pins of the semiconductor package have occurred. For example, in gate arrays in which the number of terminals is increased in proportion to the number of gates, one having 300 or more pins has already been produced.
In the manufacture of such a semiconductor package, smaller and more precise leads with a smaller lead pitch with respect to a lead frame on which an IC chip is to be mounted are required.
Conventionally, a lead frame having these small leads is formed in accordance with the etching technique. As the etching technique especially used for formation of the lead frame, a wet etching scheme is employed.
According to this wet etching scheme, as shown in FIG. 9A, belt-like resist films 2 are formed on the upper and lower surfaces of a metal plate material 1, serving as the material of the lead frame and having a thickness t, to have the same width a and the same interval b as those of leads 3 to be formed.
An etchant is sprayed on the two surfaces of the metal plate material 1 to chemically dissolve and thus remove the metal plate material 1, as indicated by arcs A in FIGS. 9B to 9D, except portions where the resist films 2 are formed.
When etching is ended, the resist films 2 are removed by washing, thereby forming the leads 3 having a width a and an interval b, i.e., a pitch P=(a+b), as shown in FIG. 9E.
In the wet etching scheme, as etching progresses, a phenomenon so-called side-etching occurs wherein etching progresses also in the lateral direction of the leads 3, as shown in FIG. 9D.
When the pitch of the leads 3 to be formed is to be decreased, the etching depth (same size of the thickness t) is increased when compared to the etching width (the interval b of the adjacent leads 3), thereby accelerating the side-etching phenomenon. Therefore, it is conventionally difficult to form the leads 3 to have the width a and the interval b that are smaller than the thickness t of the metal plate material 1, and the pitch P=(a+b) of the leads 3 becomes twice or more the thickness of the metal plate material 1.